The present invention relates to a radio-frequency (RF) power level control circuit for use in a mobile radio telephone, and more particularly to a circuit capable of being applied to a vehicle radio telephone by controlling a booster module thereof, with utilizing an RF power control system of hand-held mobile radio telephone.
Generally, the mobile radio telephone includes a vehicle radiophone (hereinafter referred to as car phone) and a portable hand-held phone (hereinafter referred to as HHP). The mobile radio communication system for operating the car phone and HHP employs a so-called cellular radio communication method. The cellular radio communication method divides its entire service area into a plurality of sub-areas called cells and employs a different frequency for each of the cells, wherein so far as there is no frequency interference between the cells, the frequency band used in other cells may be employed again to increase the communication efficiency and capability and to improve the communication quality. The known cellular communication method may be represented by the AMPS (Advanced Mobile Phone Service ) system developed by the Bell Laboratory or ARTS (American Radio Telephone Service) system developed by AT & T. A mobile radio communication system includes a mobile telephone switching office, cell sites, and mobile radiophones. The mobile telephone switching office communicates with the cell sites on wire, while the cell site communication with the mobile radiophone is wireless. The mobile telephone switching office interconnects the wire telephone line network with the mobile radio communication system, and controls the channels allotted for the cell sites. The cell site controls various signals dealing with the wire telephone network in cooperation with the mobile telephone switching office, monitors the signal intensity of mobile radiophones within its own cell area to provide the information of the signal intensity to the mobile telephone switching office, and transfers to mobile radiophones the information received from the mobile telephone switching office. Thus, telephone subscribers connected to the mobile telephone switching office can communicate with a selected mobile radiophone. Upon using this system, if RF-signal power of a certain mobile radiophone near the cell site is controlled to increase, the communication sensitivity to the other mobile radiophone distant from the cell site will decrease. Hence, the cell site monitors the signal sensitivity of the mobile radiophone within its cell in order to adjust the signal sensitivity. To achieve the improvement of communication sensitivity, the mobile radio telephone system prescribes the effective radiation power (ERP) according to the classification of the mobile radiophones, as shown in the following table 1.
TABLE 1 ______________________________________ EIA Specification (Power Output and Power Control) Power Level MAC ERP(dBw) according to Power Class (PL) I II III ______________________________________ PL0 000 6 2 -2 PL1 001 2 2 -2 PL2 010 -2 -2 -2 PL3 011 -6 -6 -6 PL4 100 -10 -10 -10 PL5 101 -14 -14 -14 PL6 110 -18 -18 -18 PL7 111 -22 -22 -22 ______________________________________
Therefore, by monitoring the signal sensitivity of the mobile radiophone, the a cell site outputs the mobile attenuation code (MAC) signal of three bits in order to control the signal intensity. Receiving the MAC signal, the mobile radiophone radiates the RF signal for the Power Level of a given class. The Power Class is determined according to the classes of the mobile radiophone as shown in Table 1, for example, the car phone is classified into class I, and a HHP is classified into class III. Thus, the total number of power levels for the car phone is 8, and each level is reduced by 4-dB starting from the highest level (PL0). On the other hand, the total number of power levels for HHP is 6 (since, PL0-PL2 are common), and each level is reduced by 4-dB starting from the highest level (PL0-PL2).
However, since the largest output level of such a HHP is about 0.6-watt and it further decreases upon use in a vehicle, the probability of communication success therein becomes low. Thus, when trying to employ the HHP in a vehicle, an output level thereof has to be amplified over 3-watts, that is, the maximum output level of the car phone. To overcome such a drawback, there is employed a booster capable of amplifying an output of the HHP up to close to that an of the car phone and making the communication by taking an antenna designed for use in the car phone, with gain of which is at least three times as high as that of an HHP-antenna. In such a construction, the HHP is disposed on a driver's seat, and the booster module is disposed in the trunk of a car. In addition, the HHP and the booster module are coupled to each other through a high-frequency cable of preferably 50-ohms, in which the booster module takes its power source from a battery of a car. As shown in the above table 1, while the number of RF-power levels in the HHP is six since three power levels PL0, PL1 and PL2 are the same, the number of RF-power levels of the car phone is eight. Hence, when the HHP of known arts is to be used as the mobile radio telephone for vehicles, an additional control circuit should be adapted, so that the RF control output according to class III of the HHP could be applied to that of class I of the car phone, within the booster module